Inputting amount of zoom manually
A convenient way to do this doesn't seem to be available. But you can lock the camera to the view and then change the camera's position.


Press Num0 to view the scene from the active camera
(optional) Press Home to maximize the camera region in the viewport
In the 3D View's properties shelf select View > Lock Camera to View




Zooming with the mouse does not change the camera's focal length but its position. You should be able manually change the “zoom level” by scaling the camera towards / from the object that you're looking at.

As far as I can tell the camera scale is not considered for the values of Depth of Field, but as a precaution one can enable transformation lock for the scale value of the camera.